Tonic adaptation in symptomatic and asymptomatic subjects.

Tonic accommodation and tonic vergence were measured in symptomatic and asymptomatic subjects before, during, and after 45 min of binocular reading at near (5 D). Reading produced significant increases in both tonic levels, with the magnitude of change being similar in the two groups. After reading, both groups exhibited rapid loss of the tonic vergence effect, but showed no evidence for decay of the tonic accommodation effect. Differences between the groups were observed in the rate of adaptation during the course of reading, with both tonic components increasing more slowly for the symptomatic subjects than for the asymptomatic subjects. In addition, baseline tonic accommodation was significantly higher in the symptomatic group than in the asymptomatic group. The results suggest that although the failure to undergo tonic oculomotor adaptation is not a causal factor in asthenopia, the initial steady-state level and/or subsequent temporal changes of tonic accommodation and tonic vergence may be related to nearwork symptomatology.

Near work, visual fatigue, and variations of oculomotor tonus.

One hour of near work produced adaptive changes in the resting states of accommodation and binocular vergence, which may be responsible for different aspects of visual fatigue. Two groups of college students read from either hard copy or a video display terminal (VDT). Immediately before and after reading, the subjects' distance acuity, dark vergence, dark focus, and accommodative response accuracy for a monocular stimulus were measured. After reading, subjects also rated their subjective feelings of visual fatigue. Reading produced significant changes in both accommodation and vergence, which did not differ for the hard copy and VDT modes of presentation. Dark focus and accommodative responses shifted in the myopic direction by an average of 0.6 D and at least 0.35 D, respectively; dark vergence distance shifted in the convergent direction by an average of 11.4 cm. These changes were greatest for subjects whose initial resting postures corresponded to a far distance. After reading, one third of the subjects exhibited lower visual acuity at distance. This change was significantly correlated with changes in dark focus (r = 0.35) but not with changes in dark vergence (r = -0.12). In contrast, subjective ratings of visual fatigue were not correlated with changes in the dark focus (rho = 0.13), but they were significantly correlated with changes in dark vergence (rho = 0.58). These findings indicate that ordinary near work can produce significant changes in the resting states of accommodation and vergence, whose magnitude depend on the subject's initial oculomotor resting tonus.(ABSTRACT TRUNCATED AT 250 WORDS)